The id is our unique identifier for a headline/url combination.
Its how we can keep track of which headlines we have seen before and which ones are new.
The id is either the guid from rss,
the URL or a MD5 checksum generated from the headline text (if $headline->headline_as_id is true);

A bool value that determines whether the URL will be the unique identifier or the if an MD5 checksum of the RSS title will be used instead.
(when the URL doesn't provide absolute uniqueness or changes within the RSS feed)

This is used in extreme cases when URLs aren't always unique to new healines (Use Perl Journals) and when URLs change within a RSS feed (www.debianplanet.org / debianplanet.org / search.cpan.org,search.cpan.org:80)